The measurement of longitudinal strain of optical fibers using several optical techniques is discussed. A review of the optical principles used to design each measurement system is included. Mathematical expressions for strain due to tensile stress, thermal stress, and hydrostatic pressure are provided. Each technique is based upon directly or indirectly measuring the change in the transit time of an optical signal injected into the test fiber. Equations are provided that relate the strain to the change in fiber transit time. Examples of calibration results and cable tests are given.